This invention relates to apparatus for continuously surveying and aligning railroad track.
One well-established technique, described in U.S. Pat. No. 3,411,455, Stewart and von Beckmann, involves the use of an infrared beam transmitter, two infrared receivers and two shadow boards. The transmitter typically is mounted on a self-propelled front rail engaging buggy, both receivers are mounted on a rear rail engaging buggy and one shadow board is mounted on a self-propelled jacking car positioned much nearer to the rear buggy than to the front buggy and arranged to tow the rear buggy. The other shadow board is mounted on a further buggy closely positioned in front of the jacking car and arranged to be pushed by the jacking car.
The transmitter together with the last mentioned shadow board (reference) and one of the receivers operate as a reference system to establish a reference line and the first mentioned shadow board (detecting) together with the other receiver operate as a detection system to detect the track condition relative to the reference line. More particularly, the reference line is established by moving the reference shadow board transversely outwardly to interrupt the beam from the transmitter to the reference receiver. The reference receiver and the detection receiver are mounted for conjoint movement and they also can be moved transversely so as to vary the ratio of the distance the reference shadow board projects transversely from the track to the distance the receivers extend transversely according to whether the alignment apparatus is operating on straight (tangent) track where this ratio is a fixed constant, circular track where the ratio is a different fixed constant and spiral track where the ratio varies continuously. In a practical example of the prior system only one receiver is used, the system being switched from a "detecting" mode to an "aligning" mode. For this purpose the two shadow boards have flip away edges permitting only the appropriate shadow board to work with the receiver at any one time. In any event, a human operator has to decide what type of track is being operated on.
The detection system indicates when the track at the working station, where the detection shadow board is located, is out of alignment with the reference line. More particularly if the detection shadow board blocks the beam from the transmitter to the detection receiver or single receiver, the receiver signals a jack on the jacking car to move the track a sufficient distance to permit the receiver to "see" the beam.
One disadvantage of such a system is that it requires a human operator to make decisions based on judgment and expertise in order to arrive at a preadjustment of the apparatus.
Other systems which overcome this disadvantage have been proposed. For example U.S. Pat. No. 4,176,456, Helmuth von Beckmann, describes a system in which, instead of a shadow board technique, two overlapping mechanical chords, which may be wires or rods, are provided. A first measuring device is located at a predetermined point on one of the chords and measures the lateral distance of that point on the one chord from the track centre line. Processing circuitry is arranged to sum and average distance values sampled at ten or so consecutive points each spaced apart two meters or so such that a running mean track position value or reference is obtained.
A second measuring device is located at another predetermined point on the other chord and measures the lateral distance of that point from the track center line. The value obtained is compared with the mean value or reference obtained from the processing circuitry and any difference or error causes a track correcting device located adjacent the second measuring device to move the track rightwards or leftwards to reduce the error.
U.S. Pat. No. 4,166,291, Charles Shupe, describes a similar system in which three mechanical chords are used instead of two and the measuring devices measure the angles between each successive pair of chords rather than offset from the chords. The principle is otherwise the same as that taught in U.S. Pat. No. 4,176,456.
Both of the latter two described systems suffer from the disadvantage that a wire or rod serving as a mechanical chord passes by the position at which the track correcting machinery is located tending to obstruct proper operation of the correcting machinery or damage to the chord. This is particularly true if an attempt is made to operate in switches where the correcting machinery has to move laterally in order to cover a branch line track.